Battery chargers are commonly used for charging battery operated electronic devices such as battery operated pagers, cellular telephones and radios. Modern chargers used for charging such communication devices are typically capable of charging at different charge rates. For example, a typical battery charger can charge both at a rapid-charge rate and a trickle charge rate depending on the condition of the battery. A charger will usually first determine the capacity of the battery to be charged by sensing a code resistor in the battery which informs the charger of the capacity of the battery. The charger then begins to charge the battery using a predetermined charge rate based on the battery's capacity. While charging the battery, the charger senses the temperature of the battery by using a thermistor which is part of the battery. If the battery begins to heat up beyond predetermined limits, the battery charger lowers the rate the battery is being charged at in order to prevent the battery from being damaged.
One problem with present day chargers is that in order to be able to change charge rates and determine the temperature of the battery requires the charger to have a costly on-board controller such as a microprocessor, or costly discrete components to perform the control function. Another technique employed by prior art chargers such as that disclosed in U.S. Pat. No. 5,115,182, entitled "Battery Charging Controller For A Battery Powered Device And Method For Using The Same", by Ehmke et al., is for either the charger to provide several current outputs and have the battery powered device select the appropriate current source, or have the electronic device generate several current sources from a single charger output with the battery powered device again selecting the appropriate current source.
Another problem with prior art charging systems is that while an electronic device such as a radio is being charged, it can change state from a standby state to a state in which the unit is operational, such as the case of a cellular telephone that is being charged while the telephone user is operating the unit. The problem with the radio changing states while it is being charged is that more current is required by the unit in the operational state then when it is in the standby mode. If the battery charger can not compensate for this increase need for current immediately, the unit may begin to consume energy from its battery if the unit is requiring more current then the charger is providing. A need thus exists for a charging system which can automatically compensate for a units change in current requirements while being charged, and also for a charger which does not require the expense of an on-board control unit.